In a telescope, binoculars, a microscope, and other optical apparatus, an objective lens forms a real image and an ocular lens further magnifies the real image for observation. In an ocular lens of this type, aberrations are, of course, required to be satisfactorily corrected over a wide angle of view, and a sufficient eye relief (on-axis distance between lens surface of ocular lens that is closest to a viewer's eye and eye point of ocular lens) is also required for comfortable observation. In general, when an apparent field of view of an ocular lens is increased, however, it is well known that a sufficiently long eye relief is difficult to provide and aberrations associated with light fluxes at the periphery of the visual field, particularly, curvature of field and astigmatism, sharply increase.
To address the problem, for example, an ocular lens including a negative lens group disposed on the object side, a positive lens group disposed on the viewer's eye side, and a field stop disposed therebetween has been disclosed (Patent Literature 1, for example). In the thus configured ocular lens disclosed in Patent Literature 1, disposing the negative lens group on the object side allows a long eye relief to be provided. Further, when the negative lens group has large refracting power (power), the Petzval sum can be reduced, whereby curvature of field is satisfactorily corrected. That is, in an ocular lens of this type, the Petzval sum can be reduced when the ratio of the focal length of the negative lens group on the object side to the focal length of the positive lens group on the viewer's eye side approaches 1:1, and the reduction in the Petzval sum is advantageous in correcting curvature of field and other aberrations.